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A Zoo of Geometric Homology Theories

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A Zoo of Geometric Homology Theories A zoo of geometric homology theories Matthias Kreck May 8, 2018 1 Introduction A homology theory is on the one hand given by a spectrum - and from this point of view homology theories are almost as general as spaces. Originally they occurred in a completely different form by geometric constructions like simplicial or singular homology theories or later bordism theories, K-theory (a cohomology theory) and others. In this note we introduce a zoo of homology theories which both generalize singular homology and bordism theory in a natural way. More precisely for each subset A of N A N the natural numbers we construct a homology theory h∗ which for A = −{1} is ordinary singular homology and for A = {0} is singular bordism. The theories in our zoo are all bordism groups, which generalize the case of smooth manifolds by allowing singularities. There are many concepts of manifolds with singularities one could use here. For our pupose the objects the author introduced some years ago, which are called stratifolds, work particularly well [3]. The theory of stratifolds was further elaborated in [2] in the thesis of the author’s PhD student Anna Grinberg. The zoo comes from forcing certain strata indexed by the subset A to be empty. Despite their simple construction computations of these groups seem to be very complicated. We give a few simple examples. Thus there are no interesting applications so far and the zoo looks a bit like a curiosity. But one never knows for what these theories might be good in the future. We mention a concrete question which might be useful in connection with the Griffith group consisting of algebraic cycles in a smooth algebraic variety over the complex numbers which vanish in singular homology. I dedicate these notes to my friend Egbert Brieskorn. Egbert is (in a very different way like our common teacher Hirzebruch) a person which had a great influence on me. When I had to make a complicated decision I often had him in front of my eyes and asked myself: What would Egbert suggest? arXiv:1805.02409v1 [math.AT] 7 May 2018 Conversations with him were always intense and fruitful. I miss him very much. When I thought about a subject for this note I also asked myself, what would Egbert say about this or that mathematics. I have no idea what he would say about this zoo. But I hope he would at least like the occurrence of manifolds with singularities. And it would probably find his interest that if Y is a compact complex singular variety in a non-singular complex algebraic variety X it admits a natural structure of a stratifold with all odd-dimensional strata empty and so represents a homology class in the special case where A consists of all odd numbers. 2 Generalized homology theories and singular bordism To motivate the construction let me recall the definition of singular bordism groups. Let X be a topo- logical space. Then a cycle is a pair f : M → X, where M is a closed smooth n-dimensional manifold and f a continuous map. Two cycles (M,f) and (M ′,f ′) represent the same bordism class if and only if there is a compact manifold W with ∂W = M + M ′, and an extension F : W → X of the maps f and f ′. This is an equivalence relation and the equivalence classes form a group under disjoint union denoted by Nn(X), the n-th singular bordism group. If g : X → Y is a continuous map it induces a homomorphism g∗ : Nn(X) → Nn(Y ) 1 given by post-composition and this way we obtain for each n a functor from the category of topological spaces to the category of abelian groups. By construction (using the cylinder as a bordism) this is a ′ ′ homotopy functor, meaning that if g and g are homotopic, then g∗ = g∗. This functor is a homology theory, which normally is expressed as an extension to the category of topological pairs fulfilling the Eilenberg-Steenrod axioms. But an equivalent simple characterization is the following. As in the case of relative homology groups one has to add data to a functor h∗, namely a boundary operator, which in our case is the boundary operator for a Mayer-Vietoris sequence: for open subsets U and V a natural operator d : hk(U ∪ V ) → hk−1(U ∩ V ) Then a homology theory is a homotopy functor h∗ together with a natural boundary operator as above, such that the Mayer-Vietoris sequence ... → hk+1(U ∪ V ) → hk(U ∩ V ) → hk(U) ⊕ hk(V ) → hk(U ∪ V ) → .... is exact. Here the maps are given by the boundary operator, the induced maps of the inclusions and the difference of the induced maps of the inclusions. Examples of homology theories are singular homology and the bordism groups N∗(X). In this case the boundary operator is given a follows. If f : M → U ∪ V is a continuous map, then consider A := f −1(U) and B := f −1(V ). These are disjoint open subsets. Thus there is a smooth function ρ : M → R, which on A is 0 and on B is 1. Let t ∈ (0, 1) be a regular value of f. Then d[(M,f)] is represented by −1 f|f −1(t) : f (t) → U ∩ V . The construction of singular bordism was carried out in [1] on the category of pairs of spaces. The proof that our absolute bordism theory is a homology theory uses the same ideas. It has nothing to do with the fact that the cycles are maps on smooth manifolds. If works identically for manifolds with singularities, where it was worked out in [3]. The same arguments apply to the generalized bordism theories constructed below. 3 Stratifolds There are plenty of definitions of stratified spaces, starting from Whitney stratified spaces over Mather’s abstract stratified spaces, which both are differential topological concepts, to purely topological concepts. All of them is common that it is a topological space together with a decomposition into manifolds, which are called strata. Since we want to generalize bordism of smooth manifolds we restrict ourselves to differential topological stratifolds. Our approach to stratifolds is motivated by a definition of smooth manifolds in the spirit of algebraic geometry as topological spaces together with a sheaf of functions, which in the traditional definition corresponds to the smooth functions. Then a manifold is a Hausdorff space M with countable basis together with a sheaf C of continuous functions, which is locally diffeomorphic to Rn equipped with the sheaf of all smooth functions. Here a morphism between spaces X and X′ equipped with subsheaves of the sheaf of smooth functions is a continuous map f such that if ρ′ is in the sheaf over X′, then ρ′f is in the sheaf over X. An isomorphism or here called diffeomorphism is a bijetive map f such that f vand f −1 is a morphism. Having this in mind it is natural to generalize this by considering topological Hausdorff spaces S with countable basis together with a sheaf C of continuous functions, such that for f1, ..., fk in C and k f a smooth function on R , the composition f(f1, .., fk) is in C. A stratifold is defined as a pair (S, C) such that the following properties are fulfilled. Given C one can define the tangent space TxS at a point x ∈ S as the vector space of all derivations of the germ Γx(C) of smooth functions at x. This gives a k decomposition of S as subspaces S := {x ∈ S| dimTxS = k},. These subspaces are called the k-strata of S. The union of all strata of dimension ≤ k is called the k-skeleton Σk. Definition 1. An n-dimensional stratifold is a pair (S, C) as above such that k (1) For all k the stratum S together with the restriction SSk of the sheaf to it is a smooth manifold, i.e. 2 locally diffeomorphic to Rk. (2) All skeleta are closed subsets of S. (3) The strata of dimension >n are empty. (4) For each x ∈ S and open neighborhood U there is a so called bump function ρ : S → R≥0 in C, such that suppρ ⊂ U and ρ(x) > 0. k (5) For each x ∈ S the restriction gives an isomorphism Γx(C) → Γx(C|Sk ). A continuous map f : S → S′ is called a morphism or smooth, if fρ ∈ C for each ρ ∈C′. If f is a homeomorphism and f and f ′ are smooth, the f is called a diffeomorphism. A smooth map f induces, as for smooth manifolds, a linear map between the tangent spaces, the differential. It is given by pre-composition with the map f mapping a derivation at x ∈ S to a derivation of S′ at f(x). This induced map is called the differential of f at x. Whereas the other conditions are natural, one might wonder where the last condition comes from. If one looks at Mather’s abstract stratified spaces, then he gives the decomposition of the space into the strata plus additional data. Amongst them there are neighborhoods of the strata together with retracts π to the strata. Then Mather defines smooth (also called controlled) functions f as continuous functions such that for each stratum the restriction to it is smooth and there is a smaller neighborhood such that π restricted to the smaller stratum commutes with f. This implies our condition (5) and actually one can reconstruct π from our data, if (5) is fulfilled ([3],p.
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